1. Field of the Invention
The claimed technical solution relates to methods for generating radiation at resonance transitions of metal atoms in electric-discharge excited mixtures of inert gases and metal vapors and may be of interest for applications in photochemistry, microelectronics, ecology (purification of water and air), lighting engineering (fluorescent lamps) and other fields.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Electronic transitions between the ground state and first excited states of metal atoms are, potentially, highly efficient sources of narrow-band radiation in the ultraviolet (UV) and visible spectrum fields, when discharge-excited. Radiation in the UV is generated, first of all, by excited mercury atoms (wavelengths of resonance transitions 61P1-61S0, 63P1-61S0 are, approximately, 185 nm and 254 nm), the known Na duplet 52P1/2-32S1/2 (589.6 nm) and 52P3/2-32S1/2 (589.0 nm) corresponds to the visible spectrum. The resonance radiation of these two metal atoms is of main interest and is frequently used in applications.
In this connection, methods of exciting the above-said transitions in electric discharges (glow and arc discharges, those of low and high pressures, etc.) have been studied in detail; the most efficient parameters have been achieved for low-pressure arc discharges in mixtures of mercury or sodium atoms and inert gases. The excitation of resonance states and the discharge optical properties for mercury and sodium (as well as for vapors of other metals) are similar ([1]: Rokhlin, G. N. “Discharge Sources of Light”. 2nd Edition, reworked and supplemented.—M.: Energoatomizdat, 1991, 720 pp.). A detailed description of the optical properties of discharge in mercury and a mixture of mercury and argon, that takes into account radiation reabsorption, is presented, for example, in the review ([2]: Fabricant, V. A. “Some Issues of gas discharge optics”. UFN, 1947, V. 32, Issue 1, pp. 1-25). It has been shown in experiments and theoretical calculations [1,2] that under optimal conditions the efficiency of exciting low resonance and metastable levels by an electric discharge may reach approx. 75% for mercury atoms and more than 80% for sodium atoms, which makes the respective discharge metal-vapor lamps, first of all mercury-vapor lamps and sodium-vapor lamps, a prospective radiation source. Methods for exciting a discharge in such lamps are, on the whole, analogous to each other, the main differences, including structural ones, are associated with temperature at which optimal pressure of metal saturated vapors is realized.
A method for generating radiation at resonance transitions of a metal atom (mercury) is known that includes excitation of low-pressure mixtures of inert gases and metal atom by an alternating longitudinal electric discharge of industrial frequency (50 Hz) ([1]). This excitation method is used, apart from other things, in commonly known fluorescent mercury-vapor lamps [1]. As the metal atom source the known method uses metal mercury (metal sodium) kept at a certain temperature (corresponding, as a rule, to saturated metal vapor pressure of approx. 0.3÷1 Pa), and as an inert gas argon or an argon-neon mixture is used at a pressure of several hundreds of Pa. A metal-vapor lamp is, as a rule, a cylinder having a diameter ranging from 15 to 50 millimeters and a length of 0.3÷1.5 meters; the discharge operating temperature, which corresponds to the metal vapor optimal concentration, is approx. 45° C. for mercury and approx. 280° C. for sodium.
Under the above-said conditions an efficiency of generating UV-radiation by mercury atoms is rather high—a real efficiency of transforming the discharge energy into radiation of a resonance transition of a mercury atom at 254 nm may reach 25%, and a luminous efficacy of fluorescent mercury-vapor lamps according to the known method may reach 70 lm/W, that of sodium lamps—200 lm/W. The use of alternating electric current of industrial frequency (i.e., current which direction is changed for the opposite one every 10 milliseconds) for exciting a discharge enables practically completely preclude migration of positively charged ions (and, as a result, atoms) of mercury to a “instant cathode” and instability of radiation along the tube length. Furthermore, if quarts of high purity or other materials transparent in the field of 185 nm are used for making tubes of mercury-vapor lamps, up to 6% of the power involved in a discharge is emitted additionally, and, thus, a total generation efficiency of the UV-radiation in a mercury-vapor lamp according to the known method may reach approx. 30%.
But the efficiency of the known method is far from maximum possible values; furthermore, the known method is ecologically dangerous for mercury-vapor lamps, since it is necessary to recycle significant quantity of mercury which is in the liquid state in the lamp and quickly vaporizes at the room temperature, when the lamp life is over or the lamp is damaged. Furthermore, an electric discharge lights up and terminates in each half-period at 50 Hz frequency of alternating current (since, in the absence of an electric field, an electron life-time in the discharge is fractions of a millisecond), which requires relighting of a discharge in each cycle and reduces the electrode lifetime significantly, and also causes significant fluctuations of the radiation power. Moreover, when a small voltage is supplied to the lamp, the excitation efficacy of a mercury (sodium) atom is low, and energy supplied to the discharge transforms into heat practically in full due to elastic losses in collisions of electrons with atoms, which reduces not only the efficiency, but also the possible lamp radiation power (since a temperature of the lamp walls is limited due to quick rise in pressure of saturated metal vapors when temperature increases).
The closest technical solution (prototype) is a method for generating radiation at resonance transitions of metal atoms in a longitudinal high-frequency arc discharge of low pressure, which excites mixtures of inert gases and metal vapors. This method is studied most closely in respect of arc mercury-vapor lamps, first of all for amalgam mercury-vapor lamps ([3]: Kostyuchenko S. V., Kuzmenko M. Ye., Pecherkin V. Ya. “Study of operation of powerful amalgam sources of low-pressure bactericide radiation at the frequency of 40 kHz”. The electronic journal “Studied in Russia”, 2000, V. 3, pp. 1365-1372; http://zhurnal.ape.relarn.rWarticles/2000/100.pdf).
For the typical conditions of using the known method with the quasi-sinusoidal pumping frequency in the range of 30-50 kHz the main temporary parameters of discharge plasma are: circular frequency of sinusoidal excitation ω˜(2÷3)·105 Hz reverse life-time of excited atoms (considering radiated photons reabsorption at optimal pressure of metal vapors—in the known method—mercury) 1/T*˜(0.5÷1)·105 Hz, frequency of electron temperature relaxation in a discharge 1/Te˜(5÷7)·105 Hz. Excitation of a discharge at a circular frequency exceeding the reverse life-time of excited atoms (and, moreover, reverse life-time of electrons in a discharge) results in that an electron concentration does not practically change during the pumping period, and both a concentration of radiating (excited) atoms and a radiation power change within the limits of ±(20÷30)%. Due to the non-linear dependence of an excitation rate on electron temperature Te, this concentration approaches the value corresponding to maximum temperature Te during a period. At the same time a frequency of an electron temperature relaxation 1/Te in a discharge is so high that in any moment Te corresponds to an applied field, and, as a result, a concentration of radiating particles in a high-frequency discharge is ensured at lesser average electron energy, that is at a lesser value of elastic losses, and, correspondingly, at a rise in the efficiency of electric energy transformation into light.
In particular, in [3], when a longitudinal electric discharge excited one and the same mixture of the inert gases (Ar/Ne) with mercury vapors in the tube with the inter-electrode's distance of 1450 mm, the radiation generation efficiency at the wavelength of ˜254 nm was approx. 33.6% at the pumping frequency of 50 Hz and 39.5% at the pumping frequency of 40 kHz. Furthermore, at a high frequency current anode voltage drop is reduced, a lowering energy release in the anode region leads to an increase in the electrodes service life. Similarly to this, a lowering electron average temperature reduces a diffusion rate of atomic (molecular) ions to the tube walls, and bombardment of the walls by ions also defines to a considerable degree the service life of modern metal-vapor lamps.
Thus, the application of a longitudinal high-frequency quasi-sinusoidal discharge for generating radiation in an arc discharge in mixtures of inert gases and metal vapors enabled to raise its efficacy significantly, increase the service life of the electrodes and, correspondingly, the lamp on the whole, as well as ensure rather high stability of radiation in the current change period.
In the case of mercury-vapor lamps the use of amalgams as the mercury atom source radically increases the safety of these lamps. At the room temperature (and even up to 50÷60° C.) a pressure of mercury saturated vapors over the amalgams used in the mercury-vapor lamps is low, mercury in such an amalgam lamp is in the bound state practically completely, the lamp has, as the vapors, approx. 0.03 mg per lamp, and it is this quantity that may pass to the atmosphere if the lamp is broken, as compared to several milligrams of mercury (and more) in the “common” mercury-vapor lamps. Furthermore, the use of an amalgam mercury atom source enabled in the known method, without complicating the structure at the same mercury atom to raise the operation temperature of the gas mixture to approx. 100° C., as compared to approx. 45° C. in a lamp with metal mercury, i.e., increase the energy input and the linear power of the generated UV-radiation significantly.
However, the efficiency of metal-vapor lamps according to the known method is considerably lesser than potential possibilities of a low-pressure discharge in mixtures of inert gases and metal vapors, of interest is also an increase in the service life of metal-vapor lamps, first of all lamps with high linear power of radiation.